This invention relates in general to vacuum treating devices and in particular to a new and useful method of heating articles in a vacuum using a plasma flame and a magnetic force which favors the generation of the plasma acting in a direction skirting the surfaces of the articles.
The present invention relates to a method of heating articles in a vacuum container. Such methods are applied for degassing, soldering, sintering, hardening and in connection with coating and ion-beam treatment, for example. The purpose is to obtain an as uniform as possible heating of the articles. In prior art vacuum heating furnaces, the articles to be heated are, for example, surrounded by heating surfaces wherefrom the heat is transmitted by radiation or heat conduction. Electrically conducting articles may also be heated by induced currents, and further known is the use of a glow discharge as the source of heat which, in the case of so-called anomalous discharges, covers the entire cathode surface uniformly, so that the articles connected as cathode can be uniformly heated.
It is also known to heat substances, such as metals to be melted, in a vacuum through electron bombardment. In such an instance, however, a particular geometric arrangement of the source of electrons must be provided to obtain the desired temperature distribution in the heated substance. Up to now, only correspondingly high costs made it possible to ensure a uniform heating. Usually, however, the electron bombardment is provided just to an opposite effect, namely for producing narrowly limited hot areas having temperatures which greatly differ from those of the surrounding areas, for which purpose the easily focusable electron beams are particularly suitable.
A special form of heating by electron bombardment is heating by means of a low voltage arc discharge; for purposes of the present specification, by low voltage arc discharge, a gas discharge is understood which is established between a hot cathode emitting electrons by thermionic emission, and an anode (in this connection, it is irrelevant whether the cathode is kept at the emission temperature by the gas discharge alone, or through additional heating). Mostly, an inert gas is introduced at a location close to the cathode, such as into the cavity of a hollow cathode or into a special hot cathode chamber which communicates with the vacuum container through an aperture. It is customary to concentrate the plasma issuing from the hollow cathode or the hot cathode chamber and entering through the aperture into the container, by means of a magnetic field. The electrons then move along narrow helical paths whose center lines correspond to a large extent to the lines of magnetic force of the field. Arrangements of this kind are known, for example, from U.S. Pat. Nos. 3,210,454 and 4,197,175. Both these patents describe heating of a melt connected as the anode, by means of a magnetically beamed low voltage arc discharge which is directed (aimed) at the melt. This orientation of the beam is obtained by causing the lines of magnetic force, and thus the helical paths of the electrons, to extend through the mentioned aperture and then through the melt. The low voltage arc is thus employed for producing locally limited hot areas having temperatures greatly differing from those of the ambience. To employ electron beam furnaces or arc furnaces for heating articles or substances uniformly over the entire extent of their surfaces appeared difficult, as already mentioned, since a satisfactory uniformity in the electric current distribution in the heated material could hardly be achieved.